Government sources have confirmed the vote on Mrs May's withdrawal deal will take place on 15 January.

The PM's deal – which covers the terms of the UK's divorce and the framework of future relations with the EU – has already been agreed with EU leaders. But it needs to pass a vote by MPs before it is accepted.

The UK is due to leave the EU on 29 March 2019 whether the deal is passed by MPs or not.

'United on one thing'

Dame Caroline Spelman – a Remain supporter who was environment secretary for two years when David Cameron was prime minister – told BBC Radio 4's Westminster Hour programme that 209 MPs had signed the letter.

She said that "crashing out" of the EU without a deal would cause job losses.

Asked if the prime minister "gets it", Dame Caroline said: "Yes, I definitely think she gets it. She wouldn't have invited us to come in and see her if she didn't."

Dame Caroline said the signatories to her letter included Brexit and Remain supporters – but the letter did not bind them to supporting the PM's withdrawal deal.

Media playback is unsupported on your deviceMedia captionDame Caroline Spelman says 209 MPs have signed a letter urging the prime minister to rule out a 'no deal' Brexit

Instead, Dame Caroline said, it creates a "platform" which would "stabilise the economy and give reassurance to manufacturing".

"We are united on one thing – we want to protect jobs and livelihoods by making sure we don't crash out without a deal," she said.

"If Parliament gives expression to not wanting to crash out without a deal, it means effort has got to be renewed to find an agreement which is acceptable to a majority of parliamentarians.

"Taking a step back and at least agreeing we're not going to crash out without a deal means that on 29 March we're not just going to fall over the cliff edge."

Boris: 'No deal is popular'

Mrs May's deal is facing opposition from many of her own MPs, as well as Labour and other opposition parties including the Remain-supporting Liberal Democrats.

The DUP – which Mrs May's Conservative Party relies on for a majority in Parliament – has said it will not back the deal.

Many Conservative MPs continue to believe the deal does not represent the Brexit the country voted for, and some are actively calling for Britain to leave with no deal.

If the UK leaves the EU without a deal, it would automatically fall back on World Trade Organization rules – which would apply automatically to trade between the UK and EU.

Writing in the Telegraph on Monday, Brexiteer Mr Johnson said of all the options suggested, the no-deal option is "gaining in popularity" and dismissed the warnings against it which he said were "downright apocalyptic".

"In spite of – or perhaps because of – everything they have been told, it is this future that is by some margin preferred by the British public," he said.

Mr Johnson said: "We must hope that Theresa May really does remove the backstop from the withdrawal agreement, in such a way as to give real legal protection to the UK.

"Failing that, we should approach the challenge of leaving on WTO terms in a way that is realistic and sensible, but also with the optimism and self-confidence displayed by the majority of the British public."

On Sunday, Tory MP Peter Bone told Sky News the best way to "get on" with Brexit was to leave without a deal – which would be "absolutely OK".

He said support for leaving without a deal was "hardening".

But speaking on Sunday, Mrs May warned that if Parliament rejects her Brexit deal, the country faces "unchartered territory".

Media playback is unsupported on your deviceMedia captionTheresa May: "The danger… is we actually end up with no Brexit at all"

The UK's exit in March was "in danger" if MPs did not vote for it, she added.

As well as the invite to all signatories of the letter to Downing Street, Mrs May has also invited all Tory MPs to drinks receptions on Monday and Wednesday.